Rotary metal-graphite composite targets for x-ray tubes have been known and described, or example, in the U.S. Pat. No. 4,901,338, as targets having an annular graphite substrate secured to the back surface of a disk-shaped target made of a refractory metal material such as tungsten, molybdenum or related alloys such as titanium--zirconium--molybdenum (TZM). Graphite has been considered a suitable material for the manufacture of rotary anodes for x-ray tubes because graphite is resistant to acid, chemically inert and has a significantly higher heat capacity and thermal emissivity than metals. However, when used as a part of a rotary x-ray tube target, the substrate graphite has temperature about 1100.degree. C. under normal operating conditions and therefor is sensitive to its environment. Although the target is usually maintained in a vacuum environment, there are problems which are associated with the x-ray tube operation.
There is the problem with oxidation process which degrades the graphite, producing carbon monoxide and/or carbon dioxide even at relatively low temperatures below 400.degree. C. The hydration process can cause a degradation of the graphite substrate, forming a series of hydrocarbons, at relatively high temperatures above 800.degree. C. These gases, as well as the high pressures generated in the x-ray tube, have adverse effects on the useful lifetime of a tungsten filament.
It has been known in the art to protect carbonaceous materials such as graphite and carbon--carbon composites at high temperatures against oxidation by applying different coatings. Mumtaz, et al. disclosed iridium coatings on isotropic graphite (Journal of Material Science, 30, 1995, 465). Criscione, et al. disclosed iridium coatings on graphite (AIAA Journal, 4, 1996, 1791). Clift, et al. disclosed the use of Ir--Al coatings (Surface and Coatings Technology, 42, 1990, 29). However, these thick coatings for protection of carbonaceous materials created serious problems and could not be successfully utilized for manufacturing the carbon-baked x-ray tube targets. They tend to have stresses that can result in poor adhesion to the substrate, have large grains that do not provide good diffusion barrier properties, and adversely affect the emissivity of graphite substrates when optically opaque.